1. Field of the Invention
This invention relates to flame detecting apparatus.
2. Description of the Prior Art
In general, known flame detecting apparatus exploiting the electric conductivity or rectification action of a flame requires a voltage of several hundred volts to be applied between an electrode disposed in the flame position and a burner. If then a flame is present, current flows between the electrode and the burner and is amplified by an amplifier circuit of high input impedance, which employs a field-effect transistor or the like, the amplified output signal being used as a control signal to control the supply of fuel to the burner. Since, however, the impedance of the flame is very large, the current flowing in the flame is minute, and it is therefore difficult to obtain a stable control signal, or it is necessary to use a complex amplifier having a very high gain.